1. Field of the Invention
The present invention relates to an integrated switch formed of bipolar transistors.
2. Discussion of the Related Art
The simplest way of forming a switch in bipolar technology is to use a transistor in saturated state, the voltage drop thereacross (collector-emitter) being also minimum. The maximum possible power can then be transferred to a load to be supplied, series-connected, to this bipolar transistor. To place a transistor in saturation state, a given base current must be applied thereto so that the gain (ratio of the collector current to the base current) is forced to a value smaller than the minimum gain of this transistor in linear state.
A difficulty lies in the fact that, by setting a determined base current, the intrinsic switch power consumption (linked to its base current) remains constant, even for a vertical load. This especially makes this type of assembly poorly adapted to low-consumption applications.
To overcome this disadvantage, switches in bipolar technology enabling regulation of the base current of the main transistor according to the current surged by the load have already been provided.
FIG. 1 shows a conventional example of such a so-called adaptive switch.
In the illustrated example, main transistor 1 is a PNP transistor connected, in series with a load Q, between an input terminal IN on which will be applied a D.C. supply voltage Vcc and a terminal M representing the circuit's electric ground. The emitter of transistor 1 is connected to terminal IN forming an input terminal of the switch and its collector defines an output terminal OUT, connected to load Q having its other terminal at ground M.
The rest of the assembly is formed by the adaptive control circuit. This circuit is based on the copying by a transistor 2 (here, of type PNP) of a fraction of the current flowing through transistor 1. The emitter of transistor 2 is connected to terminal IN (and thus to the emitter of transistor 1), and its base is connected to that of transistor 1.
The collector of transistor 2 is connected to a current mirror, formed of two NPN-type transistors 3 and 4 (respectively defining the source transistor and the copying transistor of the mirror) having their emitters connected to ground and their respective bases interconnected to the collector of transistor 3 (and thus to the collector of transistor 2). The bases of transistors 1 and 2 are further connected to the current mirror output, on the collector of transistor 4. A biasing resistor R connects terminal IN to the bases of transistors 3 and 4.
The current drawn from the base of transistor 1 by current mirror 3–4 is Ib=Ic/(N−1)—where N represents the ratio of the emitter surface areas of transistors 1 and 2—and forces transistor 1 into a saturation state with a forced gain equal to βf=Ic/Ib=N−1. Thus, if N is chosen so that gain βf is smaller than the minimum gain of transistor 1 in linear state, the the saturation of this transistor and the switch operation of the assembly are ensured.
A NPN-type transistor 5, controlled by a two-state circuit activation signal ON/OFF, connects the collector of transistor 2 to ground. When transistor 5 conducts, the current provided by transistor 2 flows to ground and no current is then drawn from the base of transistor 1, which ensures its blocking.
A disadvantage of the structure of FIG. 1 is that transistors 1 and 2 have, between their respective collectors and emitters, different biasings. Indeed, transistor 1 operates in a saturated state with a low collector-emitter voltage while transistor 2 (unsaturated) sees across its terminals a much greater collector-emitter voltage. This collector-emitter voltage difference may induce a current copying error between the two transistors and then cause a significant increase in the switch power consumption on-load as well as in the idle state. This disadvantage more specifically appears in integrated technology where the small dimension of the components makes their parameters more sensitive to biasing conditions.
In practice, this overconsumption phenomenon linked to the difference in biasing conditions tends to be enhanced by the increase in the component's operating temperature.